NaCl Solution Environment Research Articles

Experimental Investigation of Low‐Cycle Corrosion Fatigue Behavior of AA5059 Aluminum Alloy in Air and 3.5% NaCl Solution Environments

ABSTRACTThe shipbuilding industry is increasingly adopting aluminum alloys like AA5059 over traditional steel alloys to achieve lighter structures, enhanced environmental protection, and improved energy efficiency. Ship structures are frequently subjected to fatigue loading from combined wave‐induced stresses and corrosive effects. This study investigates the low‐cycle fatigue (LCF) behavior of AA5059 aluminum alloy in both air and a 3.5% NaCl solution to assess the impact of corrosion on fatigue life. LCF tests were conducted at strain amplitudes of ΔεT/2 = 0.3%–0.7%. The findings indicate a marked reduction in fatigue life in the NaCl solution compared with air, regardless of strain amplitude. Back (σb), effective (σeff) stresses were assessed using Dickson's approach, showing reduced back stress and increased effective stress in 3.5% NaCl, indicating diminished hardening and enhanced plastic deformation. Corrosion was observed to enhance plastic strain energy density (PSED), with specimens exhibiting massing behavior in air and non‐massing behavior in the corrosive environment. Fractographic analysis revealed corrosion pits, oxide formations, and secondary cracks in the crack initiation (CI), crack propagation (CP), and final fracture (FF) regions in NaCl. These findings on the low‐cycle corrosion fatigue performance of AA5059 provide valuable guidance for its application in shipbuilding, particularly in corrosive marine environments.

Relationship between fatigue crack threshold K max,th and K 1SCC

Abstract This article describes the relationship that relates the static threshold parameter of K 1SCC (at ∼10−10 m/s) to ΔK th and K max,th (at ∼10−10 m/cycle) under fatigue load at high R-ratio like R = 0.85. At these high R-ratios (>0.75), applied K max > K 1SCC and the fracture path has a larger component of static fracture. 5083 Al alloy is adapted as an example to demonstrate this relationship at room temperature their relationship with and without GB-β anodic precipitates in inert and a 3.5 % NaCl solution environments. We are interested in the mechanical cyclic ΔK th and K max,th and quasi-static K ISCC driving forces affecting the damage in the presence of a chemical environment due to anodic dissolution of the GB-β precipitates. Detailed discussions involve how these parameters are interrelated.

Using a reverse life plot for estimating fatigue endurance/limit

Abstract This short review paper describes the use of a reverse life plot, σ a versus 1/N f , for estimating a fatigue endurance/limit, σ FL. The method is applicable for different alloy-environment systems and load R-ratios. Due to inherent scatter in the fatigue data approaching fatigue limit, a ‘staircase’ method is often utilized that requires relatively large number of specimens (around 15–30) to be tested just for fatigue endurance/limit determination alone. The proposed method uses only high-cycle-fatigue (HCF) S-N data. The estimated fatigue endurance/limit is verified against the data from staircase method for 7000 Al alloy in air and corrosive 0.5 % NaCl solution environment. The comparison with the staircase method shows fairly good agreement. An additional example shows how this method estimates endurance limits for 4140 steels tested in three environments: dry air, air with 93 % RH, and aerated 3 % NaCl solution.

Laju Korosi Logam Baja Karbon Rendah di Larutan Garam pada Berbagai Konsentrasi Inhibitor Korosi dari Ekstrak Daun Pepaya

Papaya leaf extract could be used as an organic corrosion inhibitor. Corrosion inhibitors were used as a method to reduce corrosion rate. The most corrosive environment was the marine environment. The purpose of this study was to determine the best concentration of papaya (Carica papaya) leaf extract inhibitor which could reduce the corrosion rate of low carbon steel in a 3.56% NaCl solution environment. The maceration process was carried out for 24 hours with 70% ethanol solvent with a solute:solvent ratio of 1:8 (w/v) at room temperature. Inhibitor extracts were analyzed qualitatively with the phytochemical method. The addition of FeCl3 was carried out on the inhibitor extract from papaya leaves. The color change occurred from blackish brown to greenish black which indicated the presence of tannin compounds in the papaya leaf extract. Corrosion testing was carried out with hot stream temperature setting at 45⁰C to 55⁰C and concentrations of inhibitor extracts at 300, 600, and 900 ppm. The test was conducted for 36 hours with a span of metal mass every 6 hours. Corrosion rate calculation is done by weight loss method. The results showed that papaya leaf extract inhibitors positive contained corrosion inhibitor and could reduce the corrosion rate in 3.56% NaCl solution. The corrosion rate without inhibitor is 1.023 mmpy. Corrosion rates with inhibitor concentrations of 300, 600, and 900 ppm were 0.755, 0.585, and 0.438 mmpy, respectively.

The Electroless Monolayer and Duplex Ni–B and Ni–P Coatings for 316L Stainless Steel in Synergistic Combination of Mechanical (Wear) and Chemical (Corrosion) Processes

Conventional processes are performed to improve the low hardness and low wear resistance properties of 316L steel, but these processes generally decrease the corrosion resistance. Electroless nickel alloy coatings provide a hard, wear‐resistant, and corrosion‐resistant surface. Thus, the present study aims to investigate the wear, corrosion, and tribocorrosion behaviors of monolayer and duplex coatings with nickel–boron (Ni–B) and nickel–phospore (Ni–P) on 316L steel in comparison with 316L steel in dry sliding and 0.9 wt% NaCl solution environments. It is determined that the coatings have a mixture of crystal and amorphous structures, the interfaces on the 316L are uniform, and the compatibility between the layers is good. The Ni–B, Ni–P/Ni–B, and Ni–B/Ni–P coatings are 2.3, 2.06, and 1.6 times as hard as the 316L, respectively. The wear rates of Ni–B, Ni–P/Ni–B, and Ni–B/Ni–P coatings show decrease by 99.3%, 92.5%, and 99.1% in the dry‐sliding condition and by 98.5%, 30.1%, and 19.1% in the tribocorrosion condition compared with that of 316L, respectively. It is observed that the monolayer Ni–B coating exhibits superior hardness, a higher contact angle, low electrical conductivity, and better tribological performance in both dry sliding and tribocorrosion conditions compared to the 316L and duplex coatings.

Comparison of electrochemical corrosion performance of eutectic Al-Si automotive alloy in deep seawater and 3.5% NaCl solution

The corrosion of aluminium silicon alloys is of vital importance, as they are frequently exposed to corrosive environments. The alloy is often exposed to seawater in marine applications, and despite having NaCl, the seawater contains some other ions. The purpose of this study is to compare the electrochemical corrosion performance of eutectic Al-Si alloys in seawater with 3.5% NaCl in order to understand the effects of those additional components, and an alloy with trace silicon is also considered for comparison. Electrochemical impedance spectroscopy (EIS) and Tafel polarization methods were implemented for the corrosion analysis. The results of these methods imply that the eutectic alloy is more resistant to corrosion in seawater than in NaCl solution. Some additional elements in seawater produce protective layers on the alloy surface, shielding the alloy from corrosion in seawater. The open circuit potential is shifted towards the nobler direction for the eutectic alloy than in the trace Si-added alloy. The eutectic alloy's corrosion rate and current (Icorr) are higher. Mg2Si precipitates in the eutectic alloy, forming oxide films of MgO and SiO2, protect the alloy from further corrosion. The alloys' surface morphologies were analyzed not only by optical microscope but also by scanning electron microscope. The photos depict reduced damage on the surfaces corroded in the seawater environment compared to the NaCl solution environment. The Si phases are discernable in these images. Additional demolishment and pit formations are identified on the surfaces of the eutectic alloy from SEM inspection. Fewer pits and damages are observed in the case of seawater environments. The EDS analysis shows a higher amount of oxygen for the alloy corroded in NaCl solution, supporting the phenomenon of more damage than in seawater.

The Wet–Dry Cycling Corrosion Behavior of Low-Carbon Medium Manganese Steel Exposed to a 3.5% NaCl Solution Environment

The Wet–Dry Cycling Corrosion Behavior of Low-Carbon Medium Manganese Steel Exposed to a 3.5% NaCl Solution Environment

The effects of harsh aging environments on the properties of neat and MWCNT reinforced epoxy resins

In this study, the effects of different harsh aging environments on the chemical and mechanical properties of neat and multiwall carbon nanotubes (MWCNT) reinforced epoxy resins were investigated. The neat and 1 wt% MWCNT reinforced epoxy resins were exposed to four different corrosive environments, namely deionized water, NaCl solution (10 wt% NaCl), H2SO4 solution (10 wt% H2SO4), and HCl solution (10 wt% HCl. The mass variation results revealed that the highest absorption rates were found in specimens aged in both acidic environments. The highest amount of absorption was observed in the H2SO4 environment, measured approximately 3% for neat epoxy, and 6.5% for MWCNT reinforced epoxy resin. FT-IR spectrums showed that the most affected peaks belong to specimens aged in both acidic environments. Tensile test results revealed that the aging process in H2SO4 solution and deionized water deteriorated the strength of specimens. According to the literature, it was reported that the strength of specimens aged in NaCl solution environment is decreased. However, the same trend has not been in this study. In our study, the strength of specimens aged in NaCl solution environment increased impressively by 29%, which makes this study more striking. Another important point analyzed from the tensile test results was that the MWCNT nanoparticle reinforcement adversely affects the strength of the epoxy resin. So, it can be clearly stated that the use of expensive MWCNTs as filler of epoxy resin is not useful to improve its aging resistance. The scanning electron microscope (SEM) and optical microscope images showed that acidic environments result in different fracture mechanisms from typical polymer damaged surfaces reported in available researches.

Enhancement of physico-chemical and anti-corrosive properties of tung oil based polyurethane coating via modification using anhydrides and inorganic acid

Tung oil (TO) based polyol was synthesized through hydroxylation and alcoholysis. Modification of synthesized polyol (purified alcoholysed hydroxylated tung oil (PAHTO)) was carried out using the definite proportion of phthalic anhydride (PA), maleic anhydride (MA) and boric acid (BA) acting as modifiers as well as cross-linkers. Modified polyurethane (PU) was prepared by reacting the modified polyol with toluene diisocyanate (TDI) in NCO/OH = 0.7 ratio. Spectroscopic studies of modified PU confirmed the modification process. Anti-corrosive properties of the modified PU on galvanized steel (GS) was found better than mild steel (MS) in aqueous (3.5 wt%) NaCl solution environment. The resultant surface hydrophobicity and roughness characterization along with its morphology were analyzed using the atomic force microscopy (AFM) and contact angle (CA). Chemical resistance properties, as well as the physical properties such as gloss, scratch hardness, adhesion and impact resistance, were evaluated using standard methods. It was observed that the cross-linking moiety in the backbone of the PU chain exhibits better physico-chemical properties compared to the unmodified PU. The thermal stability of modified PU coating was examined by differential scanning calorimetry (DSC) and thermal gravimetric analysis (TGA) which showed that thermal properties like decomposition and glass transition of the materials increase with PA and BA molar ratio. The potentiodynamic and electrochemical impedance spectroscopy (EIS) study inferred better corrosion resistance property of PU with the incorporation of PA and BA which prepare itself for several industrial and commercial applications.

Effect of post weld heat treatment and TiAlSiN coating on the tensile strength of autogenous plasma arc welding of duplex/ super austenitic stainless steels

The present study shows the effect of post weld heat treatment (PWHT) and TiAlSiN coating on tensile, bending strength, hardness and corrosion of Duplex Stainless Steel (DSS) and Super Austenite Stainless Steel (SASS) joined materials in plasma arc welding. Direct aging PWHT process was conducted for 32 h at 918 °C on DSS - SASS materials. The hardness value reduced at the heat affected zone of PWHT samples due to the formation of pentagonal and hexagonal precipitates. TiAlSiN coating was applied in dissimilar material at 450 °C by Physical Vapor Deposition (PVD) process with pressure 3.0 × 10−3 Pa. In case of TiAlSiN coated sample, the tensile failures occurred at the SASS where as in case of DSS - SASS, PWHT the tensile failure occurred at the fusion zone. This investigation also showed the exhaustive mechanical property relationships of Autogeneous Plasma Arc (A-PA) weldments conducted on the fusion zone based on the process of DSS - SASS, PWHT and TiAlSiN coating by employing scanning electron microscopy, X-ray diffractometry and transmission electron microscopy techniques. Potentio dynamic polarization studies were carried out on both dissimilar and coated samples in 3.5% NaCl solution environment and the results showed that the corrosion resistance of coated samples was improved. When compared with DSS - SASS and PWHT samples, the absence of CrN in TiAlSiN coated materials resulted in high tensile strength and an increase in elongation at fracture surface compared to the coated samples.

Tribo-corrosion behaviour of chromium carbide based coatings deposited by HVOF

Carbide based coatings deposited by HVOF are one of the most common solutions for wear protection of mechanical components. Among them, Cermet constituted by Cr carbide in Ni–Cr matrix is widely used in applications where corrosion risk is high, because of their ability to form a passivation layer on the surface. On the other hand, the presence of the carbide particles can act as a preferential attack site of the corrosion.In the present work, the synergistic effect of corrosion and wear on Cr carbide Cermet coatings was investigated.Coatings were deposited by HVOF (High Velocity Oxy Fuel). Their microstructure, and chemical and phase composition were characterised by SEM-EDS. Coated samples were submitted to pin on disc test, to evaluate their tribological behaviour. Other samples were submitted to electrochemical tests in order to evaluate their corrosion resistance in a NaCl solution environment. Combined effects of wear and corrosion were investigated by carrying out tribocorrosion tests on coated samples, achieving potentiodynamic polarisation curves during pin on disc tests. After testing, wear tracks and corroded surfaces were characterised by SEM.The experiments confirmed the better corrosion resistance of Cermet coatings with respect to the AISI 304 substrate, also in the presence of wear. Moreover, a significant dependence of the wear and of the COF (coefficient of friction) from the corrosion potential was observed. In particular, the COF value was about 0.3 when the test was carried out under cathodic conditions, and about 0.1 under anodic polarisation.Such a behaviour could be attributed to the passivation layer grown on the wear surface. Further experiments are needed to confirm the proposed mechanism.

Free volume characterization of sulfonated styrenic pentablock copolymers using positron annihilation lifetime spectroscopy

A series of sulfonated styrenic pentablock copolymers was characterized using positron annihilation lifetime spectroscopy (PALS). Ortho-positronium (o-Ps) lifetime was measured as a function of the polymer's degree of sulfonation and the environment in which the polymer was equilibrated. Samples ranged from an un-sulfonated styrenic pentablock copolymer to one with an ion exchange capacity (IEC) of 2.0meq/g(dry polymer), and the samples were equilibrated in either air having 50% relative humidity, de-ionized (DI) water, or 2000mgL−1 aqueous NaCl solution environments. The o-Ps lifetime, and thus, the average free volume element size, decreased as the polymer's degree of sulfonation increased and as the amount of water sorbed into the polymer increased, similar to reported decreases in o-Ps lifetime accompanying increased water sorption in uncharged cross-linked poly(ethylene glycol) hydrogels. The o-Ps lifetimes of the DI water equilibrated acid counter-ion form polymers were quite similar to those of the salt solution equilibrated sodium counter-ion form polymers, suggesting that the PALS data are not particularly sensitive to the counter-ion form of these hydrated polymers. Previously reported water and salt permeability data in polymers were correlated with the PALS data. Correlating water and salt permeability with free volume data, estimated using o-Ps lifetime, was complicated by the micro-phase separated nature of the block copolymers. The water permeability of polymers whose hydrophilic micro-domains are believed to be well connected correlated with free volume data, based on o-Ps lifetime. Salt permeability, however, did not correlate with free volume data estimated from o-Ps lifetime, according to free volume theory. In general, contributions of the hydrophobic micro-domain free volume to the PALS measurement frustrate the correlation of water and salt transport properties with PALS parameters in a manner that is not observed in other sulfonated and uncharged polymers.

Corrosion Behavior of Ultra Fine Grain Pure Magnesium for Automotive Applications

The contemporary challenge in automotive industry is weight reduction. Compared to conventional automotive materials, commercially pure magnesium is having enhanced strength-to-mass ratio and low density, which confers to optimal requirement for automotive applications. Amongst various other reasons such as low hardness, low corrosion resistance also prevents it from entering mass production .severe plastic deformation by equal channel angular extrusion (ECAE) is regarded as one of the prominent method to increase the mechanical properties of magnesium. However, the investigation in corrosion resistance of extruded magnesium in automotive environment is not discussed in depth. This paper discusses the microstructure and corrosion characteristics of commercially pure magnesium which has been extruded by ECAE at 473k through the routes a and bc up to eight passes and their three surfaces (extrusion direction, normal direction & transverse direction) in detail. The corrosion experiment has been performed using conventional electrochemical techniques such as open-circuit potential measurements and potentiodynamic polarization method in 3.5 wt% Nacl solution environment. The surface morphology of the magnesium billet was examined in detail using scanning electron microscopy (SEM) and the hardness was measured using micro Vickers hardness tester.

Study on stress corrosion crack of austenitic stainless steel in simulated ocean environment

Recent studies on stress corrosion cracking properties of 321 bend pipe and pipe with weld, 316NG austenitic stainless steel pipe in ocean environment in virtue of slow strain rate tensile stress corrosion crack test are performed in different concentration of NaCl solution. Elongation and max tensile stress are selected as parameters to evaluate the SCC susceptibility based on the phenomenon that elongation and max stress fall as samples being exposed to NaCl solution environment, the results show that 316NG has the best stress corrosion crack resistance and 321 bend pipe materials behavior oppositely. In addition, the influence of differing strain rate and concentration of NaCl solution on properties of materials are researched, elongation increases along with tensile strain rate enhancement, mechanical properties are immune to variety of Cl- concentration from 3.5% to 26.5%.

A critical consideration for the use of Al-cladding for protecting aircraft aluminum alloy 2024 against corrosion

The necessity of involving Al cladding as a means of protecting the aircraft aluminum alloy 2024 against corrosion has been assessed experimentally. Bare as well as Al-clad protected 2024 specimens were pre-corroded and then subjected to mechanical tests. By recognizing the lack on validated concepts for the laboratory simulation of the corrosion damage expected in the service conditions of an aircraft structure, in the present work two different laboratory corrosion tests, namely the immersion of the alloy in a 3.5% NaCl solution, and the exfoliation corrosion test have been employed for pre-corroding the specimens. For the case of the aggressive exfoliation corrosion environment the protective role of Al-cladding is essential for ensuring the good tensile and fatigue behavior of the 2024 alloy. On the other hand, the results have shown that for the case of the mild 3.5% NaCl solution environment, the use of a cladding layer downgrades the fatigue behavior of the material, thus giving raise to doubt on the need of involving cladding. The results demonstrate the need for systematic efforts for the development of concepts which will allow the laboratory simulation of corrosion accumulated in service.

Corrosion fatigue behavior of extruded magnesium alloy AZ61 under three different corrosive environments

Corrosion fatigue process of extruded AZ61 magnesium alloy has been investigated under three different corrosive environments: (a) high humidity environment (80% relative humidity), (b) 5 wt% NaCl solution environment, and (c) 5 wt% CaCl 2 solution environment. The fatigue strength drastically reduced under the three different corrosive environments: the reduction rates of fatigue limit under high humidity, NaCl and CaCl 2 environments were 0.22, 0.85 and 0.77, respectively. The drastic reduction in fatigue limit under corrosive environments resulted from pit formation and growth to the critical size for fatigue crack nucleation. It is suggested that the NaCl environment enhances pit formation and growth more than the CaCl 2 environment, due to the high Cl − concentration and low pH value. It is also found that the corrosion pit grows during fatigue cycles and the fatigue crack starts when the pit reaches a critical size for crack nucleation. The critical size is attained when the stress intensity factor range reaches the threshold value. The ratio between pit growth life to fatigue crack nucleation and total fatigue life was about 30%.

High Cycle Fatigue Behavior of Magnesium Alloys under Corrosive Environment

The high cycle fatigue characteristics of magnesium alloys under low humidity, high humidity (80% RH) and sprayed 5%NaCl solution environments have been introduced. Fatigue limit of bulk magnesium alloy was significantly reduced even under high humidity condition, while other structural materials such as steel and aluminum alloy showed no influence of humidity on fatigue limit. The reduction of fatigue limit under 5% NaCl environments was much larger than that under high humidity environment. The remarkable reduction of fatigue limit under corrosive environments was attributed to the formation of corrosion pit, which was induced by simultaneous action of mechanical loading and corrosive environment. To improve the reduced fatigue strength under corrosive environment, coating used to apply on the surface. Non-chromium chemical conversion coating showed superior effect on the improvement of fatigue strength under corrosive environment compared to anodized coating. Fatigue strengths of the coated and painted AZ61 alloy under high humidity and 5%NaCl environments showed almost the same fatigue strength as bulk material under low humidity.

Corrosion resistance of the polymer matrix hard magnetic composite materials Nd–Fe–B

The paper presents the characteristics of the protective polymer, lacquer, and metal coatings put down onto the hard magnetic composite substrate with the polymer matrix, reinforced with the Nd–Fe–B particles with additions of metal powders. Examinations of the coatings’ structures were made with the scanning electron microscopy method. Hardness, thickness, and adhesion tests of the deposited coatings were also made. It was found out that the protective coatings are characteristic of the uniform deposition on the entire surface and of the same thickness all over their area. These coatings are characterised by a compact structure with no visible delaminations and defects. Polymer coatings demonstrate higher hardness compared to the lacquer ones. The highest hardness of 350s is characteristic of the polyester polymer coating. It was found that the highest adherence of 8.4 MPa is characteristic of the polyester–epoxy coating, whereas, the lowest adherence is displayed by the phthal-urethane coating—5.0 MPa. The paper presents also the corrosive wear of the hard magnetic composite materials with the polymer matrix, reinforced with the Nd–Fe–B particles from the rapid quenched strip, sintered Nd–Fe–B magnets, and composite materials coated with the protective polymer, lacquer, and metal coatings. The corrosive resistance values in the water environment and in the 5% NaCl solution environment were determined. It was found out that the composite magnets with the polymer matrix demonstrate better corrosive resistance than the sintered magnets; whereas, the best protection from the corrosive environment is provided by the polymer coatings. The neural network model for evaluation of the rate of corrosive wear of the polymer matrix hard magnetic composite materials with addition of metallic powder was established based on the research results from the investigations carried out in two corrosive environments. Three types of input data were used in the investigation: the contribution of the added powder, the nominal variable that defined the corrosive environment and the time duration of the test. The percentage corrosive wear of the surface was the output produced from such input data.

Overload effects on corrosion fatigue crack initiation life and life prediction of aluminum notched elements under variable amplitude loading

In this study, corrosion fatigue tests under both constant and variable amplitude loading are carried out to investigate the overload effect on the corrosion fatigue crack initiation (CFCI) life and life prediction model of the notched specimens of LY12CZ aluminum alloy sheets, which has the continuous strain hardening characteristics. Analysis of test results of CFCI life in 3.5% NaCl solution environment shows that overload increases the CFCI threshold but has no appreciable effect on the CFCI resistance coefficient, and thus increases the CFCI life. Further analysis gives the overload effect factor for CFCI life, z, which can be used to characterize the load interaction effect in load spectrum. Furthermore, three load spectra are designed and used for testing and predicting the CFCI lives under variable amplitude loading (VAL). Test results of CFCI life under above mentioned load spectra show that the load pattern has great effect on CFCI life of LY12CZ alloy notched elements even the difference among the load spectra is small. It means that the overload effect should be taken into account in CFCI life prediction under VAL. Finally, the equivalent stress amplitude approach is put forward to predict the CFCI life under VAL by using Miner’s rule and the CFCI life expression considering the overload effect. The predicted CFCI lives of LY12CZ alloy notched elements under VAL agree well with test results. Based on the above test results and analysis, it may be thought that Miner’s rule can be used to predict the CFCI life of notched elements of metals with continuous strain hardening characteristics if the overload effect on CFCI life is taken into account in life prediction under VAL.

Effect of corrosive environment on the fatigue crack initiation and propagation behavior of Al 5454-H32

Fatigue behavior of aluminum alloy 5454- H32 was studied under laboratory air and 3 % NaCl solution environment using smooth cylindrical and notched plate specimens. Presence of 3 % NaCl environment during fatigue loading drastically reduced alloy fatigue life. The deleterious effect was pronounced in both types of specimens in the long- life regions, where the fatigue lives were lowered by as much as a factor of 10. However, the sharply notched specimens showed only a modest reduction in fatigue life in corrosive environment. The severe influence of the corrosive environment in the long- life (low- stress) regime cannot be explained merely by the early initiation of the fatigue crack from surface pits; the environmental contribution in the early crack growth regime must also be considered an important factor. Fracture surface studies revealed extensive pitting and some secondary cracking in the crack initiation region. It was shown that lowered fatigue life in Al 5454- H32 occurs by early initiation of fatigue cracks from surface pits. In addition, a corrosion pitting and secondary cracking process may be operative in the small crack growth region. This could have enhanced the early crack growth rate and thus contributed to the lower fatigue life in the long- cycle region.